The Current Status of Stereotactic Body Radiation Therapy in Kidney Cancer

Renal cancers are common, accounting for an estimated 65,340 new diagnoses and 14,970 attributable death in 2018 in the United States.1 The “Epidemiology and Etiology of Kidney Cancer” is discussed at length in the linked article in the UroToday Center of Excellence series. Despite a large number of histologic tumor types that may occur in the kidney, renal cell carcinoma (RCC) is the most prevalent histology and this article will focus on patients with RCC.

There are a number of accepted treatment options for patients diagnosed with localized RCC. These include radical nephrectomy (whether open, laparoscopic or robotic), partial nephrectomy (whether open, laparoscopic, or robotic), surgical or non-surgical ablation, and active surveillance. The most appropriate treatment strategy will depend on patient (host) and tumor characteristics. These details are discussed more fully in the “Malignant Renal Tumors” article in the UroToday Center of Excellence series.

Kidney cancer has been historically thought of as a “radio-resistant” tumor. This is based on in vitro studies2 as well as the fact that early trial of adjuvant and neoadjuvant radiotherapy in patients with RCC undergoing surgical resection failed to show benefit.3,4  As a result, traditionally fractionated radiotherapy has been historically limited to palliative intent for patients with RCC. However, hypofractionated, high-dose radiotherapy has proven successful in the local control of RCC metastasis to the brain and other bony and visceral sites (refs 6-15). Coinciding with these clinical data was the emergence of data demonstrating the efficacy of high dose per fraction radiotherapy in the treatment of RCC in a mouse model.5 This led to increasing interest in the use of stereotactic body radiotherapy (SBRT) in the treatment of localized RCC. SBRT is routinely used for the treatment of malignancies of other tissue types including lung, liver, spine, and prostate.6 Compared to other radiation techniques, SBRT utilizes a smaller number of higher dose fractions. This is believed to assist with overcoming the previously believed radioresistance of RCC. Further, compared to other ablative approaches, one of the advantages of SBRT is the ability to treat larger lesions.6

Given uncertainties about both the efficacy and toxicity of such an approach, initial investigation has focused on patients in whom extirpative surgery, the gold standard approach, is not feasible or safe.

There are currently both retrospective and prospective reports characterising outcomes for patients treated with SBRT for localized RCC. These studies include a variety of treatment approaches including single fraction treatment (often 26 Gy in 1 fraction) and multiple fraction regimes (including regimes ranging from 2 to 10 fractions and total doses ranging from 5 to 85 Gy). As may be expected from some different treatment approaches, there are differences in both efficacy and toxicity between studies.

Prospective cohort studies

A recent systematic review identified eight published prospective studies of SBRT in the treatment of patients with localized RCC.7 Apart from one study published in 2006, the remainder have been published in the last five years. The strength of conclusions that can be drawn from these data are limited by small sample sizes (4 to 40 patients with localized RCC per study) and limited follow-up (13 to 52 months, with most 2 years or less).7 In addition, as previously mentioned, there were significant differences in total dose delivered and radiotherapy prescription between studies.

In each case, the authors report on patients who were either deemed medically inoperable, at very high risk for surgery due to the risk of dialysis or who refused surgery. Some studies had specific, disease-related criteria (e.g. a single lesion, maximal tumor dimension less than 4 or 5 cm) whereas this was not specified in other manuscripts. Outcomes were variably reported with local control most often reported. Additionally, adverse events were variously, and non-systematically reported.

Local control rates varied, in large part in correlation to the duration of observation: from 87% local control rate at a median 37 months follow up to 100% at two years in one trial8. Notably, even in the publication from Siva and colleagues who reported 100% local control, 10% of patients experienced distant progression, an outcome that is more likely to contribute to morbidity and mortality than local recurrence.8 Longer-term outcomes remain to be assessed.

Likely due in part to differing radiotherapy prescriptions, toxicity rates varied significantly. Both Siva and colleagues and Svendman and colleagues reported grade 1-2 toxicity in more than 50% of patients, most notably characterized by chest wall pain, nausea, and fatigue.8,9 In addition to considerations regarding comorbidity, SBRT and other non-surgical approaches to renal masses are often considered in patients with poor renal function for whom nephron preservation is a top priority. Thus, post-procedural renal function is an important outcome and, again, results vary between reports. Kaplan and colleagues reported worsening of renal function in 2 of 12 patients undergoing SBRT for medically inoperable tumors less than 5cm.10 McBride et al., in a similar population of patients, found that 2 of 15 patients (13%) had late grade 3 renal dysfunction with a mean decrease in glomerular filtration rate of 18 mg/dL among the whole study population.11 Similarly, Ponsky and colleagues demonstrated an 11% rate of grade 3 renal dysfunction among 19 patients deemed poor surgical candidates who received SBRT.12 Finally, and perhaps more optimistically, Siva and colleagues found in their cohort of 21 patients that the average decrease in glomerular filtration rate was only 8.7 mL/min at one year following treatment.8. Taken together, evidence suggests that increased fractionation (as in 20 to 30 Gy in 10 fractions) was strongly correlated with renal atrophy.13

Taken together, these data suggest that, for patients who receive three radiation fractions, a minimum per fraction dose of 11 Gy should be administered as this was the minimum dose that, in prospective cohorts, no patient experienced local failure.7 

Retrospective cohort studies


In addition to the aforementioned prospective cohort studies, there are a number of retrospective cohort studies examining the use of SBRT in primary RCC. These, for the most part, have the same limitations are the prospective studies including limited sample size, short follow-up and heterogeneity of radiotherapy prescription. While most of these reports demonstrated local control rates comparable to the prospective literature (93 – 100%), one study demonstrated significantly lower local control (65%) among patients who had a history of radical nephrectomy for RCC in the contra-lateral kidney14. Those patients received 60 to 85 Gy in 5 to 7 fractions using stereotactic gamma-ray irradiation.

Patient selection

Surgery remains the mainstay of curative-intent treatment for patients with localized RCC. Ablative approaches, including SBRT, may, therefore, be considered among patients for whom surgery is contraindicated or who refuse surgery. Recent guidelines have recommended emphasizing that SBRT remains an experimental option in RCC due to the relatively limited worldwide experience and lack of long-term data.15

Treatment recommendations

The International Radiosurgery Oncology Consortium for Kidney performed a 65 item survey among eight institutions who performed SBRT for primary RCC.16 A number of important conclusions came out of this work and the resulting consensus statement. First, all included centers treat patients with solitary kidneys or pre-existing hypertension. Five of the eight institutions have size cut off criteria ranging from 5 to 8 cm in maximal tumor dimension. The total planning target volume expansion varied between institutions, ranging from 3 to 10 mm. While all centers used pretreatment image verification, seven of the eight utilized intrafractional monitoring of some sort. Radiation prescriptions varied from 1 to 12 fractions with a total dose of 25 to 80 Gy. However, the consensus statement recommends a total dose of 36 to 45 Gy for patients receiving 3 fraction regimes and 40 to 50 Gy for patients receiving 5 fraction regimes. Obviously, the size of the primary tumour and its proximity to critical and adjacent structures will influence the total dose and fractionation regime.

Ongoing surveillance follow-up for local tumor response and recurrence varied with some institutions relying on computed tomography (CT) alone while others used magnetic resonance imaging (MRI) or PET-CT. Typically, follow-up was performed every three to six months in the first two years and every three to twelve months in the subsequent three years.

One of the challenges in the post-treatment monitoring of these patients is the interpretation of radiographic studies and identification of imaging studies. Among 41 tumours treated with SBRT, the largest available study of imaging characteristics following treatment found that the linear growth rate regressed by an average of 0.37 cm per year after treatment but that there were no significant changed in enhancement when comparing imaging before and following treatment.17

Conclusions


Stereotactic body radiotherapy is an emerging treatment approach for patients with primary renal cell carcinoma. Compared to other ablative approaches (such as radiofrequency ablation or cryotherapy), it offers the opportunity to treat larger tumors and potentially those in closer proximity to critical structures. To date, surgical extirpation via partial or radical nephrectomy remains the gold standard and SBRT has primarily been investigated among patients who are either deemed medically inoperable or who refuse surgery. There are a number of ongoing studies assessing the role of SBRT that will increase the prospective global experience with this approach, however, none will provide comparative data with other treatment approaches. One particular area of interest in the potential for synergistic effects between SBRT and systemic therapy, particularly immunotherapy (ClinicalTrials.gov identifiers: NCT01896271, NCT02781506, NCT02306954, and NCT02334709).
Written by: Christopher J.D. Wallis, MD PhD and Zachary Klaassen, MD MSc
References: References:
1. Siegel RL, Miller KD, Jemal A. Cancer statistics, 2018. CA Cancer J Clin 2018; 68(1):7-30.
2. Deschavanne PJ, Fertil B. A review of human cell radiosensitivity in vitro. Int J Radiat Oncol Biol Phys 1996; 34(1):251-66.
3. Finney R. The value of radiotherapy in the treatment of hypernephroma--a clinical trial. Br J Urol 1973; 45(3):258-69.
4. Juusela H, Malmio K, Alfthan O, et al. Preoperative irradiation in the treatment of renal adenocarcinoma. Scand J Urol Nephrol 1977; 11(3):277-81.
5. Walsh L, Stanfield JL, Cho LC, et al. Efficacy of ablative high-dose-per-fraction radiation for implanted human renal cell cancer in a nude mouse model. Eur Urol 2006; 50(4):795-800; discussion 800.
6. Francolini G, Detti B, Ingrosso G, et al. Stereotactic body radiation therapy (SBRT) on renal cell carcinoma, an overview of technical aspects, biological rationale and current literature. Crit Rev Oncol Hematol 2018; 131:24-29.
7. Miccio J, Johung K. When Surgery Is Not an Option in Renal Cell Carcinoma: The Evolving Role of Stereotactic Body Radiation Therapy. Oncology (Williston Park) 2019; 33(5):167-73, 177.
8. Siva S, Pham D, Kron T, et al. Stereotactic ablative body radiotherapy for inoperable primary kidney cancer: a prospective clinical trial. BJU Int 2017; 120(5):623-630.
9. Svedman C, Sandstrom P, Pisa P, et al. A prospective Phase II trial of using extracranial stereotactic radiotherapy in primary and metastatic renal cell carcinoma. Acta Oncol 2006; 45(7):870-5.
10. Kaplan ID, Redrosa I, C. M, et al. Results of a phase I dose escalation study of stereotactic radiosurgery for primary renal tumors. Int J Radiat Oncol Biol Phys 2010; 78:S191.
11. McBride SM, Wagner AA, Kaplan ID. A phase 1 dose-escalation study of robotic radiosurgery in inoperable primary renal cell carcinoma. Int J Radiat Oncol Biol Phys 2013; 87:S84.
12. Ponsky L, Lo SS, Zhang Y, et al. Phase I dose-escalation study of stereotactic body radiotherapy (SBRT) for poor surgical candidates with localized renal cell carcinoma. Radiother Oncol 2015; 117(1):183-7.
13. Yamamoto T, Kadoya N, Takeda K, et al. Renal atrophy after stereotactic body radiotherapy for renal cell carcinoma. Radiat Oncol 2016; 11:72.
14. Wang YJ, Han TT, Xue JX, et al. Stereotactic gamma-ray body radiation therapy for asynchronous bilateral renal cell carcinoma. Radiol Med 2014; 119(11):878-83.
15. Muller AC, van Oorschot B, Micke O, et al. [German S3 guideline for renal cell carcinoma : Presentation and discussion of essential aspects for the radiation oncologist]. Strahlenther Onkol 2018; 194(1):1-8.
16. Siva S, Ellis RJ, Ponsky L, et al. Consensus statement from the International Radiosurgery Oncology Consortium for Kidney for primary renal cell carcinoma. Future Oncol 2016; 12(5):637-45.
17. Sun MR, Brook A, Powell MF, et al. Effect of Stereotactic Body Radiotherapy on the Growth Kinetics and Enhancement Pattern of Primary Renal Tumors. AJR Am J Roentgenol 2016; 206(3):544-53.

Epidemiology and Etiology of Kidney Cancer

Kidney cancer is a broad, encompassing term that borders on colloquial. While most physicians are referring to renal cell carcinoma when they say “kidney cancer”, a number of other benign and malignant lesions may similarly manifest as a renal mass. Considering only the malignant causes, kidney cancers may include renal cell carcinoma, urothelium-based cancers (including urothelial carcinoma, squamous cell carcinoma, and adenocarcinoma), sarcomas, Wilms tumor, primitive neuroectodermal tumors, carcinoid tumors, hematologic cancers (including lymphoma and leukemia), and secondary cancers (i.e. metastases from other solid organ cancers).

Epidemiology

In the United States, cancers of the kidney and renal pelvis comprise the 6th most common newly diagnosed tumors in men and 10th most common in women.1 In 2018, an estimated 65,340 people will be newly diagnosed with cancers of the kidney and renal pelvis in the United States. In men, this comprises 42,680 estimated new cases in 2018 representing 5% of all newly diagnosed cancers. In women, 22,660 new cases are anticipated in 2018 representing 3% of all newly diagnosed cancers. Additionally, 14,970 people are expected to die of kidney and renal pelvis cancers in 2018 in the United States, with this being the 10th most common cause of oncologic death among men.

In Europe, results are similar. In 2018, the incidence of kidney cancer is estimated at 136,500 new cases representing 3.5% of all new cancer diagnoses.2 This corresponds to an estimated age standardized rate (ASR) of 13.3 cases per 100,000 population. As in the United States, the incidence of kidney and renal pelvis cancers is higher among men (incidence 84,9000, 4.1% of all cancers, ASR 18.6 per 100,000) than women (incidence 51,600, 2.8% of all cancers, ASR 9.0 per 100,000). Correspondingly, 54,700 people were estimated to die of kidney and renal pelvis cancers in Europe in 2018, accounting for 2.8% of all oncologic deaths. The age standardized mortality rate was 4.7 deaths per 100,000 population. Again, death from kidney and renal pelvis cancer was more common among men (mortality 35,100, 3.3% of oncologic deaths, ASR 7.1 per 100,000) than among women (mortality 19,600, 2.3% of oncologic deaths, ASR 2.7 per 100,000). Interestingly, within Europe, there is considerable variation in the incidence and mortality of kidney and renal pelvis cancer between countries.2

While the aforementioned data have already demonstrated that gender is strongly associated with the risk of both diagnosis of and death from kidney and renal pelvis cancers, age also importantly moderates this risk. Among patients in the United States, the probability of developing kidney and renal pelvis cancer rises nearly ten fold from age <50 to age >70 years.1

table 1 epidemiology kidney cancer2x
Thus, kidney cancer is predominantly a disease of older adults, with the typical presentation being between 50 and 70 years of age. However, over time, rates of diagnosis of kidney cancer have increased fastest among patients aged less than 40 years old.3

In the United States, kidney cancers are more common among African Americans, American Indians, and Alaska Native populations while rates are lower among Asian Americans.4 Worldwide, the highest rates are found in European nations while low rates are seen in African and Asian countries.4

The vast majority of patients have localized disease at the time of presentation. According to Siegel et al., 65% of all patients diagnosed with kidney and renal pelvis tumors between 2007 and 2013 had localized disease at the time of presentation while 16% had regional spread and 16% had evidence of distant, metastatic disease.1 This is in large part due to incidental diagnosis due to the increased use of ultrasonography and computed tomography in patients presenting with abdominal distress. In fact, 13 to 27% of abdominal imaging studies demonstrate incidental renal lesions unrelated to the reason for the study5 and approximately 80% of these masses are malignant.6 Dr. Welch and colleagues demonstrated elegantly that the use of computed tomography is strongly related to the likelihood of undergoing nephrectomy, likely due to detection of renal masses. Thus, with the increasing utilization of abdominal imaging, the incidence of kidney cancer has increased by approximately 3 to 4% per year since the 1970s.

Renal Cell Carcinoma

Renal cell carcinoma (RCC) is the most common kidney cancer. A number of histological subtypes have been recognized including conventional clear cell RCC (ccRCC), papillary RCC, chromophobe RCC, collecting duct carcinoma, renal medullary carcinoma, unclassified RCC, RCC associated with Xp11.2 translocations/TFE3 gene fusions, post-neuroblastoma RCC, and mucinous tubular and spindle cell carcinoma. Conventional ccRCC comprises approximately 70-80% of all RCCs while papillary RCC comprises 10-15%, chromophobe 3-5%, collecting duct carcinoma <1%, unclassified RCC 1-3%, and the remainder are very uncommon.

Histologically, most of these tumors are believed to arise from the cells of the proximal convoluted tubule given their ultrastructural similarities. Renal medullary carcinoma and collecting duct carcinoma, relatively uncommon and aggressive subtypes of RCC, are believed to arise more distally in the nephron.

Familial RCC Syndromes

While the vast majority of newly diagnosed RCCs are sporadic, hereditary RCCs account for approximately 4% of all RCCs. Due in large part to the work of Dr. Linehan and others, the understanding of the underlying molecular genetics of RCC have progressed significantly since the early 1990s. These insights have led to a better understanding of both familial and sporadic RCCs.

Von Hippel-Lindau disease is the most common cause of hereditary RCC. Due to defects in the VHL tumor suppressor gene (located at 3p25-26), this syndrome is characterized by multiple, bilateral clear cell RCCs, retinal angiomas, central nervous system hemangioblastomas, pheochromocytomas, renal and pancreatic cysts, inner ear tumors, and cystadenomas of the epididymis. RCC develops in approximately 50% of individuals with VHL disease. These tumors are characterized by an early age at the time of diagnosis, bilaterality, and multifocality. Due in large part to improved management of the CNS disorders in VHL disease, RCC is the most common cause of death in patients with VHL.

Hereditary papillary RCC (HPRCC) is, as one would expect from the name, associated with multiple, bilateral papillary RCCs. Due to an underlying constitutive activation of the c-Met proto-oncogene (located at 7q31), these tumors also present at a relatively early age. However, overall, these tumors appear in general to be less aggressive than corresponding sporadic malignancies.

In contrast, tumors arising in hereditary/familial leiomyomatosis and RCC (HLRCC), due to a defect in the fumarate hydratase (1q42-43) tumor suppressor gene, are typically unilateral, solitary, and aggressive. Histologically, these are typically type 2 papillary RCC, which has a more aggressive phenotype, or collecting duct carcinomas. Extra-renal manifestations include leiomyomas of the skin and uterus and uterine leiomyosarcomas which contribute to the name of this sydrome.

Birt-Hogg-Dube, due to defect in the tumor suppressor folliculin (17p11), is associated with multiple chromophobe RCCs, hybrid oncocytic tumors (with characteristics of both chromophobe RCC and oncocytoma), oncocytoma. Less commonly, patients with Birt-Hogg-Dube may develop clear cell RCC or papillary RCC. Non-renal manifestations include facial fibrofolliculomas, lung cysts, and the development of spontaneous pneumothorax.

Tuberous sclerosis, due to defects in TSC1 (located at 9q34) or TSC2 (16p13), may lead to clear cell RCC. More commonly, it is associated with multiple benign renal angiomyolipomas, renal cystic disease, cutaneous angiofibromas, and pulmonary lymphangiomyomatosis.

Succinate dehydrogenase RCC, due to defects in the SDHB (1p36.1-35) or SDHD (11q23) subunits of the succinate dyhydrogenase complex, may lead to a variety of RCC subtypes including chromophobe RCC, clear cell RCC, and type 2 papillary RCC. Extra-renal manifestations including benign and malignant paragangliomas and papillary thyroid carcinoma. In general, these tumors exhibit aggressive behaviour and wide surgical excision is recommended.

Finally, Cowden syndrome, due to defects in PTEN (10q23) may lead to papillary or other RCCs in addition to benign and malignant breast tumors and epithelial thyroid cancers.

Etiologic Risk Factors in Sporadic RCC

While numerous hereditary RCC syndromes exist, they account for only 4% of all RCCs. However, many sporadic RCCs share similar underlying genetic changes including VHL defects in ccRCC and c-Met activation in papillary RCC. A number of modifiable risk factors associated with RCC have been described.4

The foremost risk factor for the development of RCC is cigarette smoking. According to both the US Surgeon General and the International Agency for Research on Cancer, observational evidence is sufficient to conclude there is a causal relationship between tobacco smoking and RCC. A comprehensive meta-analysis of western populations demonstrated an overall relative risk for the development of RCC of 1.38 (95% confidence interval 1.27 to 1.50) for ever smokers compared to lifetime never smokers.7 Interestingly, this effect was larger for men (RR 1.54, 95% CI 1.42-1.68) than for women (RR 1.22, 95% CI 1.09-1.36). Additionally, there was a strong dose response relationship: compared to never smokers, men who smoked 1-9 cigarettes per day had a 1.6x risk, those who smoked 10-20 per days had a 1.83x risk, and those who smoked more than 21 per day had a 2.03x risk. A similar trend was seen among women. Notably, the risk of RCC declined with increasing durations of abstinence of smoking. Smoking appears to be preferentially associated with the development of clear cell and papillary RCC.8 In addition to being associated with increased RCC incidence, smoking is associated with more aggressive forms of RCC, manifest with higher pathological stage and an increased propensity for lymph node involvement and metastasis at presentation.9 As a result, smokers have worse cancer-specific and overall survival.9

Second, obesity is associated with an increased risk of RCC. While this risk was historically felt to be higher among women, a more recent review demonstrated no such effect modification according to sex.10 In a meta-analysis of 22 studies, Bergstrom et al. estimated that each unit increase of BMI was associated with a 7% increase in the relative risk of RCC diagnosis.

Third, hypertension has been associated with an increased risk of RCC diagnosis, with a hazard ratio of 1.70 (95%CI 1.30-2.22) in the VITAL study.11 Interestingly, in an American multiethnic cohort, this effect appeared to be larger among women (RR 1.58, 95% CI 1.09-2.28) than in men (RR 1.42, 95% CI 1.07-1.87).12 Again, as with obesity, there appears to be a dose-effect relationship between severity of hypertension and the risk of RCC diagnosis.13

Fourth, acquired cystic kidney disease (ACKD) appears to be associated with a nearly 50x increase risk of RCC diagnosis.14 ACKD occurs in patients with end-stage renal disease on dialysis. These changes are common among patients who have been on dialysis for at least 3 years.14 Interestingly, the risk of RCC appears to decrease following renal transplantation.

Finally, a number of other putative risk factors have been described. These lack the voracity of data that the aforementioned four have. Such risk factors include alcohol, analgesics, diabetes, and diet habits.4

Written by: Christopher J.D. Wallis, MD, PhD
References:

1. Siegel RL, Miller KD, Jemal A. Cancer statistics, 2018. CA: a cancer journal for clinicians 2018;68:7-30.

2. Ferlay J, Colombet M, Soerjomataram I, et al. Cancer incidence and mortality patterns in Europe: Estimates for 40 countries and 25 major cancers in 2018. European journal of cancer 2018.

3. Nepple KG, Yang L, Grubb RL, 3rd, Strope SA. Population based analysis of the increasing incidence of kidney cancer in the United States: evaluation of age specific trends from 1975 to 2006. The Journal of urology 2012;187:32-8.

4. Kabaria R, Klaassen Z, Terris MK. Renal cell carcinoma: links and risks. Int J Nephrol Renovasc Dis 2016;9:45-52.

5. Gill IS, Aron M, Gervais DA, Jewett MA. Clinical practice. Small renal mass. The New England journal of medicine 2010;362:624-34.

6. Frank I, Blute ML, Cheville JC, Lohse CM, Weaver AL, Zincke H. Solid renal tumors: an analysis of pathological features related to tumor size. The Journal of urology 2003;170:2217-20.

7. Hunt JD, van der Hel OL, McMillan GP, Boffetta P, Brennan P. Renal cell carcinoma in relation to cigarette smoking: meta-analysis of 24 studies. International journal of cancer Journal international du cancer 2005;114:101-8.

8. Patel NH, Attwood KM, Hanzly M, et al. Comparative Analysis of Smoking as a Risk Factor among Renal Cell Carcinoma Histological Subtypes. The Journal of urology 2015;194:640-6.

9. Kroeger N, Klatte T, Birkhauser FD, et al. Smoking negatively impacts renal cell carcinoma overall and cancer-specific survival. Cancer 2012;118:1795-802.

10. Bergstrom A, Hsieh CC, Lindblad P, Lu CM, Cook NR, Wolk A. Obesity and renal cell cancer--a quantitative review. British journal of cancer 2001;85:984-90.

11. Macleod LC, Hotaling JM, Wright JL, et al. Risk factors for renal cell carcinoma in the VITAL study. The Journal of urology 2013;190:1657-61.

12. Setiawan VW, Stram DO, Nomura AM, Kolonel LN, Henderson BE. Risk factors for renal cell cancer: the multiethnic cohort. American journal of epidemiology 2007;166:932-40

13. Vatten LJ, Trichopoulos D, Holmen J, Nilsen TI. Blood pressure and renal cancer risk: the HUNT Study in Norway. British journal of cancer 2007;97:112-4.

14. Brennan JF, Stilmant MM, Babayan RK, Siroky MB. Acquired renal cystic disease: implications for the urologist. Br J Urol 1991;67:342-8.